1. Field of the Invention
The invention relates generally to a light sensing panel and particularly to a light sensing liquid crystal display panel.
2. Discussion of the Related Art
Touch screens have become popular in various display applications such as PDAs and other computer products. Some touch screens are used with active matrix liquid crystal display (LCD) apparatuses. Most of the touch-enabled LCD apparatuses are based on resistive, capacitive, or inductive touch technology, which requires extra components that are typically not integrated into the LCD apparatuses, such as a touch panel. The need for extra components is disadvantageous, as it adds to the cost and size of the apparatus. Further, in some cases, the extra component (e.g., a touch panel) reduces the display transmittance, adversely affecting optical performance. Thus, effort has been geared to designing a display panel with an integrated light sensing element.
A photosensitive switch for a display panel currently requires at least two switching elements and a capacitor. An example of the integrated optical touch screens is disclosed in an article by den Boer et al. titled “Active Matrix LCD with Integrated Optical Touch Screen,” SID 03 Digest (2003). An integrated optical touch screen usually includes a light sensing element that responds to the presence or absence of light to determine the location of the touch. The light sensing elements are arranged in a matrix that is designed for determining the location of the touch.
FIG. 1 is a circuit diagram showing a conventional light sensing element in an integrated touch screen display panel. The light sensing element is disposed in a pixel region of the LCD panel. A “pixel region” is defined as a rectangular area between two consecutive data lines (e.g., DP and DP+1) and two consecutive gate lines (e.g., GQ and GQ+1).
As shown in FIG. 1, the LCD panel includes a plurality of gate lines (GL), a plurality of data lines (DL), a first power supply line (VL1), a second power supply line (VL2), and two switching units. The first of the two switching units includes a switching element (Q1) that is electrically connected to the gate line GQ and the data line DP, and a liquid crystal capacitor (CLC) and a first storage capacitor (CST1) electrically connected to the switching element (Q1). The second switching unit includes a light-sensitive switching element (T1) that is coupled to the bias voltage (VDD) through a first power line (VL1) and a read-out switching element (T2) that connects the light-sensitive switching element (T1) to a read-out line (ROL). The gate electrode of the light-sensitive switching element (T1) is coupled to a second power line (VL2), and the gate electrode of the read-out switching element (T2) is coupled to the neighboring gate line (GQ+1). A second storage capacitor (CST2) electrically couples the gate of the light-sensitive switching element (T1) to a node between the light-sensitive switching element (T1) and the read-out switching element (T2).
The light-sensitive switching element (T1) responds to the presence/absence of external light by generating a charge corresponding to the sensed amount of light. The second storage capacitor (CST2) stores the charge from the light-sensitive switching element (T1). The read-out switching element (T2) outputs the charge that is stored in the second storage capacitor (CST2) to the read-out line (ROL). The light-sensitive switching element (T1), the second storage capacitor (CST2), and the read-out switching element (T2) form what is herein referred to as a photosensitive switch. Currently, the photosensitive switch requires at least two switching elements and a capacitor.
When light is incident on the light-sensitive switching element (T1), a negative voltage and a positive voltage are applied to a second power line (VL2) and a first power line (VL1), respectively, thereby turning off the light-sensitive switching element (T1). The second power line (VL2) and the first power line (VL1) are electrically connected to a gate electrode and a drain electrode of the light-sensitive switching element (T1), respectively. A current IT1 that forms as a result of the light-sensitive switching element (T1) responding to incident light is greater than a current IT2 formed by the read-out switching element (T2).
The current IT1 charges the second storage capacitor (CST2) while the read-out switching element (T2) is turned off. The charge remains stored in the second storage capacitor (CST2) until the read-out switching element (T2) is turned on, at which point the charge flows to form the current IT2 to the read-out line (ROL).
As stated above, a neighboring gate line (GQ+1) is electrically connected to the gate electrode of the read-out switching element (T2). When a high voltage gate signal is applied to the neighboring gate line (GQ+1), the charge stored in the second storage capacitor (CST2) is output to a read-out circuit (not shown) through the read-out switching element (T2) and the read-out line (ROL). Thus, when light is incident on the light-sensitive element (T1) and a high voltage is applied to the gate line GQ+1, there is a signal on the read-out line (ROL). The light-sensitive element (T1) may be formed on an array substrate (i.e., substrate with signal lines formed thereon) of the LCD panel.
A pixel region of the array substrate has enough space to accommodate the light sensing element shown in FIG. 1. However, when a transmissive LCD apparatus or a transflective LCD apparatus includes the light sensing element, the presence of the extra circuitry results in a decreased pixel aperture.
One of the disadvantages of the integrated touch screen display panel shown in FIG. 1 is that the light sensing element contains two switching elements (e.g., thin film transistors) and one capacitor. This complicated structure of the light sensing element results in a complex array substrate circuitry, which makes the array substrate production difficult and decreases the yield. Furthermore, the highly complex circuitry makes it more likely that signal interference will form between elements in the pixel region.
An integrated optical touch screen that does not increase the cost or size of the display panel and does not decrease the performance of the display panel is desired.